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Re: [Qemu-block] [PATCH] qemu-img: Do not truncate before preallocation
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-block] [PATCH] qemu-img: Do not truncate before preallocation |
Date: |
Thu, 16 Feb 2017 18:52:51 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 03.02.2017 um 20:50 hat Nir Soffer geschrieben:
> When using file system that does not support fallocate() (e.g. NFS <
> 4.2), truncating the file only when preallocation=OFF speeds up creating
> raw file.
>
> Here is example run, tested on Fedora 24 machine, creating raw file on
> NFS version 3 server.
>
> $ time ./qemu-img-master create -f raw -o preallocation=falloc mnt/test 1g
> Formatting 'mnt/test', fmt=raw size=1073741824 preallocation=falloc
>
> real 0m21.185s
> user 0m0.022s
> sys 0m0.574s
>
> $ time ./qemu-img-fix create -f raw -o preallocation=falloc mnt/test 1g
> Formatting 'mnt/test', fmt=raw size=1073741824 preallocation=falloc
>
> real 0m11.601s
> user 0m0.016s
> sys 0m0.525s
>
> $ time dd if=/dev/zero of=mnt/test bs=1M count=1024 oflag=direct
> 1024+0 records in
> 1024+0 records out
> 1073741824 bytes (1.1 GB, 1.0 GiB) copied, 15.6627 s, 68.6 MB/s
>
> real 0m16.104s
> user 0m0.009s
> sys 0m0.220s
>
> Running with strace we can see that without this change we do one
> pread() and one pwrite() for each block. With this change, we do only
> one pwrite() per block.
>
> $ strace ./qemu-img-master create -f raw -o preallocation=falloc mnt/test 8192
> ...
> pread64(9, "\0", 1, 4095) = 1
> pwrite64(9, "\0", 1, 4095) = 1
> pread64(9, "\0", 1, 8191) = 1
> pwrite64(9, "\0", 1, 8191) = 1
>
> $ strace ./qemu-img-fix create -f raw -o preallocation=falloc mnt/test 8192
> ...
> pwrite64(9, "\0", 1, 4095) = 1
> pwrite64(9, "\0", 1, 8191) = 1
>
> This happens because posix_fallocate is checking if each block is
> allocated before writing a byte to the block, and when truncating the
> file before preallocation, all blocks are unallocated.
>
> Signed-off-by: Nir Soffer <address@hidden>
Thanks, applied to the block branch.
I'm not completely sure if doing an ftruncate() first couldn't improve
PREALLOC_MODE_FULL somewhat in some cases, but I agree that the patch
should still result in correct images.
KEvin